Representation of the GPIbα N-terminus and the effect of the OS1 peptide. The fingertips represent the N-terminal β-finger, the palm represents the concave β-sheet formed by the 8 leucine-rich repeats, and the thumb represents the β–switch (R-loop). (Left) Unoccupied GPIbα with the thumb extended to interact with the A1 domain of VWF. (Right) Binding of the OS1 peptide causes the thumb to close across the palm, thereby preventing the interaction with the A1 domain of VWF. Professional illustration by Paulette Dennis.